This invention relates generally to magnetic resonance imaging, and more particularly the invention relates to determining real time spatially localized velocity distribution in a region using magnetic resonance (MR) measurements.
Quantitative velocity measurements are important in diagnosing a range of cardiovascular diseases. In stenotic mitral and aortic valves, velocities can increase from normal levels of 1 m/s to 3-4 m/s. Congenital heart disease such as coarctation can be detected by measuring high velocity in the ascending aorta, and pulmonary hypertension can be determined by the presence of high velocity tricuspid regurgitation jets.
Heretofore, the accurate measurement and localization of blood flow velocities has been achieved with Doppler ultrasonography which has found wide application in medical diagnosis. However, a variety of MR techniques have been employed to obtain velocity information.
U.S. Pat. No. 5,309,099 to frarrazabal, Hu, and Pauly discloses MR Fourier velocity encoding (FVE) which uses signal phase to encode velocity and thus results in a 3-D data set for a 2-D velocity map. If imaging is restricted along one spatial dimension, a 2-D data set of velocity as a function of position is acquired and imaging time is greatly reduced. Instead of collecting spatial frequencies (k-space comprised of kx and ky) a plane containing velocity frequencies as a function of spatial frequencies in the direction of velocity (kvx vs. kz) is acquired and reconstructed to obtain a series of velocity versus location images, which can then be temporally sampled to produce a velocity-time image at one location.
Lukpat, Pauly, Hu, and Nishimura, MRM, 40; 603-613, 1998 disclose one shot spatially resolved velocity imaging. The technique is similar to that disclosed in the two-shot imaging of U.S. Pat. No. 5,309,099, supra, except that single shot FVE with a bipolar prewinder produces a time series of velocity vs. spatial location images. The pulse sequence diagram is shown in FIG. 1. The sequence first restricts imaging to one dimension using a selective excitation. Oscillating gradients on two axes are used to create a cylindrical excitation along the third axis. The cylindrical excitation is aligned with the vessel wall, then a sawtooth readout gradient along the excitation axis is played to record a one-dimensional image down the length of the vessel. The gradient lobes have the same area and cause a back and forth motion in the spatial frequency direction. However, the gradient also results in an oscillating, magnitude increasing first moment. The first moment of the gradient is proportional to velocity frequency. Therefore, the zero and first moments result in a bowtie-shaped trajectory through spatial frequency and velocity frequency.
The present invention is directed to FVE MR imaging which allows finer object resolution while maintaining a shorter imaging time.
In accordance with the invention, an RF excitation pulse is applied to a region of interest in the presence of two orthogonal axial gradients which create a cylindrical excitation along a third axis, which can be aligned with a blood vessel such as the aorta. After excitation of the nuclei spins, the two orthogonal gradients are removed and a readout gradient is applied along the readout axis. The readout gradient is a continuous time varying cyclical gradient, such as a sawtooth wave form. In accordance with the invention, the readout gradient has lobes of increasing width which result in k-space sampling trajectories, or spokes of a bowtie, configuration with nonuniform spacing of the spokes whereby sampling density is higher at lower velocity frequencies in k-space. The variable density trajectories can provide a finer velocity resolution and maintain a shorter imaging time.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.